This invention relates to a method of fabricating a strong seal between a ceramic and a metal alloy. More specifically, the invention relates to a method of fabricating a seal strong enough to withstand extreme conditions of temperature and pressure such as those found in the environment of a pressurized water reactor (PWR).
The use of pressurized water reactors has required the development of special instruments capable of withstanding severe steady state PWR conditions. Typical of these instruments is the liquid-level transducer, an instrument used to acquire data on transient, two-phase fluid behavior in a PWR. Each transducer is comprised in part of several electrodes. Each electrode comprises in part of coaxial center pin and outer housing, both made of an electrically conductive alloy and separated from one another by a coaxial ceramic sleeve. The ceramic sleeve terminates in a ceramic seal at the point at which the center pin extends beyond the housing. A constant current is passed through the center pin and the housing and the resulting voltage between the pin and housing is measured. When the electrode is immersed in water, the water provides a conduction path for the current so that the conductivity is high and the resulting voltage is low; conversely, when the pin is dry, the conductivity is low and the voltage is high. These transducers may be mounted at different levels on the interior of a reactor vessel to indicate the water level in the vessel. If water permeates the ceramic sleeve between the pin and the housing the voltage will always be low and the electrode will give erroneous results. Thus if the ceramic seal is defective, the electrode will fail.
Seals prepared by conventional methods have proven unsatisfactory for the harsh environment of a PWR. Seals are commonly prepared by prefiring the alloy in a reducing atmosphere such as the "burnt gas" atmosphere commonly used in the art. "Burnt gas" atmospheres are quite variable in composition; the partial pressure of oxygen generally ranges anywhere from over 10.sup.5 Pa to about 1 Pa. The prefired alloy and the ceramic are then hot-pressed and fired together in air. Under these conditions the formation of oxides on the surface of the alloy cannot be controlled, and oxides which are not conducive to bonding may grow. Seals produced in this manner often have voids in the ceramic and poor bonding between the alloy and the ceramic. These seals have failed under the high temperature and pressures characteristic of a PWR.
The poor bonding is a result of materials used in the manufacture of the electrode. The alloy used in the center pin and outer housing is typically Inconel X-750, which contains significant quantities of nickel, chromium, titanium, iron, cobalt, aluminum and silicon. By current methods, during the prefiring step the alloy forms a scale of NiTiO.sub.3 on top of a layer of NiCr.sub.2 O.sub.4. These oxides have incompatible thermal expansion characteristics, which lead to spalling along the oxide interface. Additional problems may arise during the firing step. Molten ceramic will form a strong bond with certain types of oxides, but if the molten ceramic dissolves the entire surface oxide on the alloy, the ceramic will come in direct contact with the alloy and only weak van der Waals bonds will be formed.